What rear end gears for 904?

With 3.92s ; 70=~3300 rpm@ zero-slip with 28s,
which are about the tallest tires you can put in the factory tubs. But you won't be running zero-slip at 70. You can expect 165 to 330 additional rpm. All going into the trans oil, so you probably should get yourself a trans oil cooler.

with the more typical 25.5 tires the rpm could be as high as 4000.

I ran 3.91s for a while,a long time ago, and was actually happy when they blew up.